Surface Alloy Structure Determined Using Photoelectron Diffraction & Holography
Researchers working at Beamline 7.0.1 have used photoelectron diffraction and holography to study a surface alloy of manganese and nickel on a nickel (001) substrate. Their results lend hope for x-ray photoelectron holography as a method of direct, quantitative determination of surface structures.
These diagrams show the structure of a surface alloy of manganese (green) and nickel (blue) on a nickel (001) substrate, as constructed from two different techniques. The figure on the left was constructed from low-energy and x-ray photoelectron diffraction (XPD), and that on the right comes from a simple holographic transformation of an XPD data set.
The figure on the left is more accurate than the holographic transformation, but producing it required a comparison of experimental data with existing models of the sample. The holographic transformation, on the other hand, was performed directly from experimental data without reference to a starting model.
To produce the holographic transformation, researchers directed x rays at their sample and measured the intensity of photoelectrons emitted from manganese atoms as a function of emission angle and the x-ray photon energy. Before escaping the sample surface, the electrons interacted with other atoms (scatterers) in the sample, and this interaction created angular intensity variations. The figure on the right shows the Fourier transform of the resulting data set; the transform is a map of the locations of the scatterers. The manganese emitter atom (not shown in the holographic transformation) is located at the origin. The chemical identity of the scatterer atoms (i.e., nickel) was obtained by a simple reasoning from the symmetry of the structure.
The presence of in-plane maxima at the locations shown in the holographic transformation makes it clear that the sample is a surface alloy rather than a manganese overlayer. Buckling in the surface layer also appears clearly.
This work was conducted at the ultraESCA endstation on Beamline 7.0.1 by principal investigators Sylvain Ravy (Centre National de la Recherche Scientifique/Université Paris-Sud) and Brian Tonner (University of Wisconsin-Milwaukee), with Santanu Banerjee, Jonathan Denlinger, and others from the Tonner group at UWM.
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